From Lexicanum

Penitent Engines are used by the Ecclesiarchy as a form of punishment for repenting heretics. They are used by the Adepta Sororitas like Space Marines use Dreadnoughts. Similar to Dreadnoughts, the pilot is hard-wired into the frame of the Penitent Engine. They will charge into combat in a frenzied state, made to recognise that only in death can he or she find forgiveness for his or her crimes.

Ordo Hereticus Army List
HQ	Canoness • Palatine • Inquisitor Lord • Priest
Elites	Arco-flagellants • Callidus Assassin • Celestian Squad • Culexus Assassin Death Cult Assassin • Eversor Assassin • Inquisitor • Vindicare Assassin • Repentia
Troops	Battle Sister Squad • Inquisitorial Storm Troopers • Zealots Seconded Units: IG Armoured Fist Squad • IG Infantry Platoon • SM Tactical Squad • SM Scout Squad
Fast Attack	Dominion Squad • Seraphim Squad Seconded Units: IG Rough Rider • IG Sentinel Squadron • SM Assault Squad • SM Bike Squadron • SM Land Speeder Squadron
Heavy Support	Exorcist • Immolator • Penitent Engine • Orbital Strike • Retributor Squad Seconded Units: IG Leman Russ • SM Devastators • SM Dreadnought • SM Predator
Transport	Rhino • Chimera • Immolator

Sources

• Codex, Witch Hunters